1 /*
2 * Copyright (c) 1998, 2015, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/orderAccess.inline.hpp"
36 #include "runtime/osThread.hpp"
37 #include "runtime/stubRoutines.hpp"
38 #include "runtime/thread.inline.hpp"
39 #include "services/threadService.hpp"
40 #include "trace/tracing.hpp"
41 #include "trace/traceMacros.hpp"
42 #include "utilities/dtrace.hpp"
43 #include "utilities/macros.hpp"
44 #include "utilities/preserveException.hpp"
45 #ifdef TARGET_OS_FAMILY_linux
46 # include "os_linux.inline.hpp"
47 #endif
48 #ifdef TARGET_OS_FAMILY_solaris
49 # include "os_solaris.inline.hpp"
50 #endif
51 #ifdef TARGET_OS_FAMILY_windows
52 # include "os_windows.inline.hpp"
53 #endif
54 #ifdef TARGET_OS_FAMILY_bsd
55 # include "os_bsd.inline.hpp"
56 #endif
57
58 #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64)
59 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
60 #define ATTR __attribute__((noinline))
61 #else
62 #define ATTR
63 #endif
64
65
66 #ifdef DTRACE_ENABLED
67
68 // Only bother with this argument setup if dtrace is available
69 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
70
71
72 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
73 char* bytes = NULL; \
74 int len = 0; \
75 jlong jtid = SharedRuntime::get_java_tid(thread); \
76 Symbol* klassname = ((oop)obj)->klass()->name(); \
77 if (klassname != NULL) { \
78 bytes = (char*)klassname->bytes(); \
79 len = klassname->utf8_length(); \
80 }
81
82 #ifndef USDT2
83
84 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
85 jlong, uintptr_t, char*, int);
86 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
87 jlong, uintptr_t, char*, int);
88 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
89 jlong, uintptr_t, char*, int);
90 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
91 jlong, uintptr_t, char*, int);
92 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
93 jlong, uintptr_t, char*, int);
94
95 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
96 { \
97 if (DTraceMonitorProbes) { \
98 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
99 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
100 (monitor), bytes, len, (millis)); \
101 } \
102 }
103
104 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
105 { \
106 if (DTraceMonitorProbes) { \
107 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
108 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
109 (uintptr_t)(monitor), bytes, len); \
110 } \
111 }
112
113 #else /* USDT2 */
114
115 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
116 { \
117 if (DTraceMonitorProbes) { \
118 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
119 HOTSPOT_MONITOR_WAIT(jtid, \
120 (monitor), bytes, len, (millis)); \
121 } \
122 }
123
124 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
125 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
126 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
127 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
128 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
129
130 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
131 { \
132 if (DTraceMonitorProbes) { \
133 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
134 HOTSPOT_MONITOR_##probe(jtid, \
135 (uintptr_t)(monitor), bytes, len); \
136 } \
137 }
138
139 #endif /* USDT2 */
140 #else // ndef DTRACE_ENABLED
141
142 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
143 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
144
145 #endif // ndef DTRACE_ENABLED
146
147 // Tunables ...
148 // The knob* variables are effectively final. Once set they should
149 // never be modified hence. Consider using __read_mostly with GCC.
150
151 int ObjectMonitor::Knob_Verbose = 0 ;
152 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
153 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
154 static int Knob_HandOff = 0 ;
155 static int Knob_ReportSettings = 0 ;
156
157 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
158 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
159 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
160 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
161 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
162 static int Knob_SpinEarly = 1 ;
163 static int Knob_SuccEnabled = 1 ; // futile wake throttling
164 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
165 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
166 static int Knob_Bonus = 100 ; // spin success bonus
167 static int Knob_BonusB = 100 ; // spin success bonus
168 static int Knob_Penalty = 200 ; // spin failure penalty
169 static int Knob_Poverty = 1000 ;
170 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
171 static int Knob_FixedSpin = 0 ;
172 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
173 static int Knob_UsePause = 1 ;
174 static int Knob_ExitPolicy = 0 ;
175 static int Knob_PreSpin = 10 ; // 20-100 likely better
176 static int Knob_ResetEvent = 0 ;
177 static int BackOffMask = 0 ;
178
179 static int Knob_FastHSSEC = 0 ;
180 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
181 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
182 static volatile int InitDone = 0 ;
183
184 #define TrySpin TrySpin_VaryDuration
185
186 // -----------------------------------------------------------------------------
187 // Theory of operations -- Monitors lists, thread residency, etc:
188 //
189 // * A thread acquires ownership of a monitor by successfully
190 // CAS()ing the _owner field from null to non-null.
191 //
192 // * Invariant: A thread appears on at most one monitor list --
193 // cxq, EntryList or WaitSet -- at any one time.
194 //
195 // * Contending threads "push" themselves onto the cxq with CAS
196 // and then spin/park.
197 //
198 // * After a contending thread eventually acquires the lock it must
199 // dequeue itself from either the EntryList or the cxq.
200 //
201 // * The exiting thread identifies and unparks an "heir presumptive"
202 // tentative successor thread on the EntryList. Critically, the
203 // exiting thread doesn't unlink the successor thread from the EntryList.
204 // After having been unparked, the wakee will recontend for ownership of
205 // the monitor. The successor (wakee) will either acquire the lock or
206 // re-park itself.
207 //
208 // Succession is provided for by a policy of competitive handoff.
209 // The exiting thread does _not_ grant or pass ownership to the
210 // successor thread. (This is also referred to as "handoff" succession").
211 // Instead the exiting thread releases ownership and possibly wakes
212 // a successor, so the successor can (re)compete for ownership of the lock.
213 // If the EntryList is empty but the cxq is populated the exiting
214 // thread will drain the cxq into the EntryList. It does so by
215 // by detaching the cxq (installing null with CAS) and folding
216 // the threads from the cxq into the EntryList. The EntryList is
217 // doubly linked, while the cxq is singly linked because of the
218 // CAS-based "push" used to enqueue recently arrived threads (RATs).
219 //
220 // * Concurrency invariants:
221 //
222 // -- only the monitor owner may access or mutate the EntryList.
223 // The mutex property of the monitor itself protects the EntryList
224 // from concurrent interference.
225 // -- Only the monitor owner may detach the cxq.
226 //
227 // * The monitor entry list operations avoid locks, but strictly speaking
228 // they're not lock-free. Enter is lock-free, exit is not.
229 // For a description of 'Methods and apparatus providing non-blocking access
230 // to a resource,' see U.S. Pat. No. 7844973.
231 //
232 // * The cxq can have multiple concurrent "pushers" but only one concurrent
233 // detaching thread. This mechanism is immune from the ABA corruption.
234 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
235 //
236 // * Taken together, the cxq and the EntryList constitute or form a
237 // single logical queue of threads stalled trying to acquire the lock.
238 // We use two distinct lists to improve the odds of a constant-time
239 // dequeue operation after acquisition (in the ::enter() epilog) and
240 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
241 // A key desideratum is to minimize queue & monitor metadata manipulation
242 // that occurs while holding the monitor lock -- that is, we want to
243 // minimize monitor lock holds times. Note that even a small amount of
244 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
245 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
246 // locks and monitor metadata.
247 //
248 // Cxq points to the the set of Recently Arrived Threads attempting entry.
249 // Because we push threads onto _cxq with CAS, the RATs must take the form of
250 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
251 // the unlocking thread notices that EntryList is null but _cxq is != null.
252 //
253 // The EntryList is ordered by the prevailing queue discipline and
254 // can be organized in any convenient fashion, such as a doubly-linked list or
255 // a circular doubly-linked list. Critically, we want insert and delete operations
256 // to operate in constant-time. If we need a priority queue then something akin
257 // to Solaris' sleepq would work nicely. Viz.,
258 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
259 // Queue discipline is enforced at ::exit() time, when the unlocking thread
260 // drains the cxq into the EntryList, and orders or reorders the threads on the
261 // EntryList accordingly.
262 //
263 // Barring "lock barging", this mechanism provides fair cyclic ordering,
264 // somewhat similar to an elevator-scan.
265 //
266 // * The monitor synchronization subsystem avoids the use of native
267 // synchronization primitives except for the narrow platform-specific
268 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
269 // the semantics of park-unpark. Put another way, this monitor implementation
270 // depends only on atomic operations and park-unpark. The monitor subsystem
271 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
272 // underlying OS manages the READY<->RUN transitions.
273 //
274 // * Waiting threads reside on the WaitSet list -- wait() puts
275 // the caller onto the WaitSet.
276 //
277 // * notify() or notifyAll() simply transfers threads from the WaitSet to
278 // either the EntryList or cxq. Subsequent exit() operations will
279 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
280 // it's likely the notifyee would simply impale itself on the lock held
281 // by the notifier.
282 //
283 // * An interesting alternative is to encode cxq as (List,LockByte) where
284 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
285 // variable, like _recursions, in the scheme. The threads or Events that form
286 // the list would have to be aligned in 256-byte addresses. A thread would
287 // try to acquire the lock or enqueue itself with CAS, but exiting threads
288 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
289 // Note that is is *not* word-tearing, but it does presume that full-word
290 // CAS operations are coherent with intermix with STB operations. That's true
291 // on most common processors.
292 //
293 // * See also http://blogs.sun.com/dave
294
295
296 // -----------------------------------------------------------------------------
297 // Enter support
298
299 bool ObjectMonitor::try_enter(Thread* THREAD) {
300 if (THREAD != _owner) {
301 if (THREAD->is_lock_owned ((address)_owner)) {
302 assert(_recursions == 0, "internal state error");
303 _owner = THREAD ;
304 _recursions = 1 ;
305 OwnerIsThread = 1 ;
306 return true;
307 }
308 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
309 return false;
310 }
311 return true;
312 } else {
313 _recursions++;
314 return true;
315 }
316 }
317
318 void ATTR ObjectMonitor::enter(TRAPS) {
319 // The following code is ordered to check the most common cases first
320 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
321 Thread * const Self = THREAD ;
322 void * cur ;
323
324 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
325 if (cur == NULL) {
326 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
327 assert (_recursions == 0 , "invariant") ;
328 assert (_owner == Self, "invariant") ;
329 // CONSIDER: set or assert OwnerIsThread == 1
330 return ;
331 }
332
333 if (cur == Self) {
334 // TODO-FIXME: check for integer overflow! BUGID 6557169.
335 _recursions ++ ;
336 return ;
337 }
338
339 if (Self->is_lock_owned ((address)cur)) {
340 assert (_recursions == 0, "internal state error");
341 _recursions = 1 ;
342 // Commute owner from a thread-specific on-stack BasicLockObject address to
343 // a full-fledged "Thread *".
344 _owner = Self ;
345 OwnerIsThread = 1 ;
346 return ;
347 }
348
349 // We've encountered genuine contention.
350 assert (Self->_Stalled == 0, "invariant") ;
351 Self->_Stalled = intptr_t(this) ;
352
353 // Try one round of spinning *before* enqueueing Self
354 // and before going through the awkward and expensive state
355 // transitions. The following spin is strictly optional ...
356 // Note that if we acquire the monitor from an initial spin
357 // we forgo posting JVMTI events and firing DTRACE probes.
358 if (Knob_SpinEarly && TrySpin (Self) > 0) {
359 assert (_owner == Self , "invariant") ;
360 assert (_recursions == 0 , "invariant") ;
361 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
362 Self->_Stalled = 0 ;
363 return ;
364 }
365
366 assert (_owner != Self , "invariant") ;
367 assert (_succ != Self , "invariant") ;
368 assert (Self->is_Java_thread() , "invariant") ;
369 JavaThread * jt = (JavaThread *) Self ;
370 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
371 assert (jt->thread_state() != _thread_blocked , "invariant") ;
372 assert (this->object() != NULL , "invariant") ;
373 assert (_count >= 0, "invariant") ;
374
375 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
376 // Ensure the object-monitor relationship remains stable while there's contention.
377 Atomic::inc_ptr(&_count);
378
379 EventJavaMonitorEnter event;
380
381 { // Change java thread status to indicate blocked on monitor enter.
382 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
383
384 Self->set_current_pending_monitor(this);
385
386 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
387 if (JvmtiExport::should_post_monitor_contended_enter()) {
388 JvmtiExport::post_monitor_contended_enter(jt, this);
389
390 // The current thread does not yet own the monitor and does not
391 // yet appear on any queues that would get it made the successor.
392 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
393 // handler cannot accidentally consume an unpark() meant for the
394 // ParkEvent associated with this ObjectMonitor.
395 }
396
397 OSThreadContendState osts(Self->osthread());
398 ThreadBlockInVM tbivm(jt);
399
400 // TODO-FIXME: change the following for(;;) loop to straight-line code.
401 for (;;) {
402 jt->set_suspend_equivalent();
403 // cleared by handle_special_suspend_equivalent_condition()
404 // or java_suspend_self()
405
406 EnterI (THREAD) ;
407
408 if (!ExitSuspendEquivalent(jt)) break ;
409
410 //
411 // We have acquired the contended monitor, but while we were
412 // waiting another thread suspended us. We don't want to enter
413 // the monitor while suspended because that would surprise the
414 // thread that suspended us.
415 //
416 _recursions = 0 ;
417 _succ = NULL ;
418 exit (false, Self) ;
419
420 jt->java_suspend_self();
421 }
422 Self->set_current_pending_monitor(NULL);
423
424 // We cleared the pending monitor info since we've just gotten past
425 // the enter-check-for-suspend dance and we now own the monitor free
426 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
427 // destructor can go to a safepoint at the end of this block. If we
428 // do a thread dump during that safepoint, then this thread will show
429 // as having "-locked" the monitor, but the OS and java.lang.Thread
430 // states will still report that the thread is blocked trying to
431 // acquire it.
432 }
433
434 Atomic::dec_ptr(&_count);
435 assert (_count >= 0, "invariant") ;
436 Self->_Stalled = 0 ;
437
438 // Must either set _recursions = 0 or ASSERT _recursions == 0.
439 assert (_recursions == 0 , "invariant") ;
440 assert (_owner == Self , "invariant") ;
441 assert (_succ != Self , "invariant") ;
442 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
443
444 // The thread -- now the owner -- is back in vm mode.
445 // Report the glorious news via TI,DTrace and jvmstat.
446 // The probe effect is non-trivial. All the reportage occurs
447 // while we hold the monitor, increasing the length of the critical
448 // section. Amdahl's parallel speedup law comes vividly into play.
449 //
450 // Another option might be to aggregate the events (thread local or
451 // per-monitor aggregation) and defer reporting until a more opportune
452 // time -- such as next time some thread encounters contention but has
453 // yet to acquire the lock. While spinning that thread could
454 // spinning we could increment JVMStat counters, etc.
455
456 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
457 if (JvmtiExport::should_post_monitor_contended_entered()) {
458 JvmtiExport::post_monitor_contended_entered(jt, this);
459
460 // The current thread already owns the monitor and is not going to
461 // call park() for the remainder of the monitor enter protocol. So
462 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
463 // event handler consumed an unpark() issued by the thread that
464 // just exited the monitor.
465 }
466
467 if (event.should_commit()) {
468 event.set_klass(((oop)this->object())->klass());
469 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
470 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
471 event.commit();
472 }
473
474 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
475 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
476 }
477 }
478
479
480 // Caveat: TryLock() is not necessarily serializing if it returns failure.
481 // Callers must compensate as needed.
482
483 int ObjectMonitor::TryLock (Thread * Self) {
484 for (;;) {
485 void * own = _owner ;
486 if (own != NULL) return 0 ;
487 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
488 // Either guarantee _recursions == 0 or set _recursions = 0.
489 assert (_recursions == 0, "invariant") ;
490 assert (_owner == Self, "invariant") ;
491 // CONSIDER: set or assert that OwnerIsThread == 1
492 return 1 ;
493 }
494 // The lock had been free momentarily, but we lost the race to the lock.
495 // Interference -- the CAS failed.
496 // We can either return -1 or retry.
497 // Retry doesn't make as much sense because the lock was just acquired.
498 if (true) return -1 ;
499 }
500 }
501
502 void ATTR ObjectMonitor::EnterI (TRAPS) {
503 Thread * Self = THREAD ;
504 assert (Self->is_Java_thread(), "invariant") ;
505 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
506
507 // Try the lock - TATAS
508 if (TryLock (Self) > 0) {
509 assert (_succ != Self , "invariant") ;
510 assert (_owner == Self , "invariant") ;
511 assert (_Responsible != Self , "invariant") ;
512 return ;
513 }
514
515 DeferredInitialize () ;
516
517 // We try one round of spinning *before* enqueueing Self.
518 //
519 // If the _owner is ready but OFFPROC we could use a YieldTo()
520 // operation to donate the remainder of this thread's quantum
521 // to the owner. This has subtle but beneficial affinity
522 // effects.
523
524 if (TrySpin (Self) > 0) {
525 assert (_owner == Self , "invariant") ;
526 assert (_succ != Self , "invariant") ;
527 assert (_Responsible != Self , "invariant") ;
528 return ;
529 }
530
531 // The Spin failed -- Enqueue and park the thread ...
532 assert (_succ != Self , "invariant") ;
533 assert (_owner != Self , "invariant") ;
534 assert (_Responsible != Self , "invariant") ;
535
536 // Enqueue "Self" on ObjectMonitor's _cxq.
537 //
538 // Node acts as a proxy for Self.
539 // As an aside, if were to ever rewrite the synchronization code mostly
540 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
541 // Java objects. This would avoid awkward lifecycle and liveness issues,
542 // as well as eliminate a subset of ABA issues.
543 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
544 //
545
546 ObjectWaiter node(Self) ;
547 Self->_ParkEvent->reset() ;
548 node._prev = (ObjectWaiter *) 0xBAD ;
549 node.TState = ObjectWaiter::TS_CXQ ;
550
551 // Push "Self" onto the front of the _cxq.
552 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
553 // Note that spinning tends to reduce the rate at which threads
554 // enqueue and dequeue on EntryList|cxq.
555 ObjectWaiter * nxt ;
556 for (;;) {
557 node._next = nxt = _cxq ;
558 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
559
560 // Interference - the CAS failed because _cxq changed. Just retry.
561 // As an optional optimization we retry the lock.
562 if (TryLock (Self) > 0) {
563 assert (_succ != Self , "invariant") ;
564 assert (_owner == Self , "invariant") ;
565 assert (_Responsible != Self , "invariant") ;
566 return ;
567 }
568 }
569
570 // Check for cxq|EntryList edge transition to non-null. This indicates
571 // the onset of contention. While contention persists exiting threads
572 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
573 // operations revert to the faster 1-0 mode. This enter operation may interleave
574 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
575 // arrange for one of the contending thread to use a timed park() operations
576 // to detect and recover from the race. (Stranding is form of progress failure
577 // where the monitor is unlocked but all the contending threads remain parked).
578 // That is, at least one of the contended threads will periodically poll _owner.
579 // One of the contending threads will become the designated "Responsible" thread.
580 // The Responsible thread uses a timed park instead of a normal indefinite park
581 // operation -- it periodically wakes and checks for and recovers from potential
582 // strandings admitted by 1-0 exit operations. We need at most one Responsible
583 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
584 // be responsible for a monitor.
585 //
586 // Currently, one of the contended threads takes on the added role of "Responsible".
587 // A viable alternative would be to use a dedicated "stranding checker" thread
588 // that periodically iterated over all the threads (or active monitors) and unparked
589 // successors where there was risk of stranding. This would help eliminate the
590 // timer scalability issues we see on some platforms as we'd only have one thread
591 // -- the checker -- parked on a timer.
592
593 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
594 // Try to assume the role of responsible thread for the monitor.
595 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
596 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
597 }
598
599 // The lock have been released while this thread was occupied queueing
600 // itself onto _cxq. To close the race and avoid "stranding" and
601 // progress-liveness failure we must resample-retry _owner before parking.
602 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
603 // In this case the ST-MEMBAR is accomplished with CAS().
604 //
605 // TODO: Defer all thread state transitions until park-time.
606 // Since state transitions are heavy and inefficient we'd like
607 // to defer the state transitions until absolutely necessary,
608 // and in doing so avoid some transitions ...
609
610 TEVENT (Inflated enter - Contention) ;
611 int nWakeups = 0 ;
612 int RecheckInterval = 1 ;
613
614 for (;;) {
615
616 if (TryLock (Self) > 0) break ;
617 assert (_owner != Self, "invariant") ;
618
619 if ((SyncFlags & 2) && _Responsible == NULL) {
620 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
621 }
622
623 // park self
624 if (_Responsible == Self || (SyncFlags & 1)) {
625 TEVENT (Inflated enter - park TIMED) ;
626 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
627 // Increase the RecheckInterval, but clamp the value.
628 RecheckInterval *= 8 ;
629 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
630 } else {
631 TEVENT (Inflated enter - park UNTIMED) ;
632 Self->_ParkEvent->park() ;
633 }
634
635 if (TryLock(Self) > 0) break ;
636
637 // The lock is still contested.
638 // Keep a tally of the # of futile wakeups.
639 // Note that the counter is not protected by a lock or updated by atomics.
640 // That is by design - we trade "lossy" counters which are exposed to
641 // races during updates for a lower probe effect.
642 TEVENT (Inflated enter - Futile wakeup) ;
643 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
644 ObjectMonitor::_sync_FutileWakeups->inc() ;
645 }
646 ++ nWakeups ;
647
648 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
649 // We can defer clearing _succ until after the spin completes
650 // TrySpin() must tolerate being called with _succ == Self.
651 // Try yet another round of adaptive spinning.
652 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
653
654 // We can find that we were unpark()ed and redesignated _succ while
655 // we were spinning. That's harmless. If we iterate and call park(),
656 // park() will consume the event and return immediately and we'll
657 // just spin again. This pattern can repeat, leaving _succ to simply
658 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
659 // Alternately, we can sample fired() here, and if set, forgo spinning
660 // in the next iteration.
661
662 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
663 Self->_ParkEvent->reset() ;
664 OrderAccess::fence() ;
665 }
666 if (_succ == Self) _succ = NULL ;
667
668 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
669 OrderAccess::fence() ;
670 }
671
672 // Egress :
673 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
674 // Normally we'll find Self on the EntryList .
675 // From the perspective of the lock owner (this thread), the
676 // EntryList is stable and cxq is prepend-only.
677 // The head of cxq is volatile but the interior is stable.
678 // In addition, Self.TState is stable.
679
680 assert (_owner == Self , "invariant") ;
681 assert (object() != NULL , "invariant") ;
682 // I'd like to write:
683 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
684 // but as we're at a safepoint that's not safe.
685
686 UnlinkAfterAcquire (Self, &node) ;
687 if (_succ == Self) _succ = NULL ;
688
689 assert (_succ != Self, "invariant") ;
690 if (_Responsible == Self) {
691 _Responsible = NULL ;
692 OrderAccess::fence(); // Dekker pivot-point
693
694 // We may leave threads on cxq|EntryList without a designated
695 // "Responsible" thread. This is benign. When this thread subsequently
696 // exits the monitor it can "see" such preexisting "old" threads --
697 // threads that arrived on the cxq|EntryList before the fence, above --
698 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
699 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
700 // non-null and elect a new "Responsible" timer thread.
701 //
702 // This thread executes:
703 // ST Responsible=null; MEMBAR (in enter epilog - here)
704 // LD cxq|EntryList (in subsequent exit)
705 //
706 // Entering threads in the slow/contended path execute:
707 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
708 // The (ST cxq; MEMBAR) is accomplished with CAS().
709 //
710 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
711 // exit operation from floating above the ST Responsible=null.
712 }
713
714 // We've acquired ownership with CAS().
715 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
716 // But since the CAS() this thread may have also stored into _succ,
717 // EntryList, cxq or Responsible. These meta-data updates must be
718 // visible __before this thread subsequently drops the lock.
719 // Consider what could occur if we didn't enforce this constraint --
720 // STs to monitor meta-data and user-data could reorder with (become
721 // visible after) the ST in exit that drops ownership of the lock.
722 // Some other thread could then acquire the lock, but observe inconsistent
723 // or old monitor meta-data and heap data. That violates the JMM.
724 // To that end, the 1-0 exit() operation must have at least STST|LDST
725 // "release" barrier semantics. Specifically, there must be at least a
726 // STST|LDST barrier in exit() before the ST of null into _owner that drops
727 // the lock. The barrier ensures that changes to monitor meta-data and data
728 // protected by the lock will be visible before we release the lock, and
729 // therefore before some other thread (CPU) has a chance to acquire the lock.
730 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
731 //
732 // Critically, any prior STs to _succ or EntryList must be visible before
733 // the ST of null into _owner in the *subsequent* (following) corresponding
734 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
735 // execute a serializing instruction.
736
737 if (SyncFlags & 8) {
738 OrderAccess::fence() ;
739 }
740 return ;
741 }
742
743 // ReenterI() is a specialized inline form of the latter half of the
744 // contended slow-path from EnterI(). We use ReenterI() only for
745 // monitor reentry in wait().
746 //
747 // In the future we should reconcile EnterI() and ReenterI(), adding
748 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
749 // loop accordingly.
750
751 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
752 assert (Self != NULL , "invariant") ;
753 assert (SelfNode != NULL , "invariant") ;
754 assert (SelfNode->_thread == Self , "invariant") ;
755 assert (_waiters > 0 , "invariant") ;
756 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
757 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
758 JavaThread * jt = (JavaThread *) Self ;
759
760 int nWakeups = 0 ;
761 for (;;) {
762 ObjectWaiter::TStates v = SelfNode->TState ;
763 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
764 assert (_owner != Self, "invariant") ;
765
766 if (TryLock (Self) > 0) break ;
767 if (TrySpin (Self) > 0) break ;
768
769 TEVENT (Wait Reentry - parking) ;
770
771 // State transition wrappers around park() ...
772 // ReenterI() wisely defers state transitions until
773 // it's clear we must park the thread.
774 {
775 OSThreadContendState osts(Self->osthread());
776 ThreadBlockInVM tbivm(jt);
777
778 // cleared by handle_special_suspend_equivalent_condition()
779 // or java_suspend_self()
780 jt->set_suspend_equivalent();
781 if (SyncFlags & 1) {
782 Self->_ParkEvent->park ((jlong)1000) ;
783 } else {
784 Self->_ParkEvent->park () ;
785 }
786
787 // were we externally suspended while we were waiting?
788 for (;;) {
789 if (!ExitSuspendEquivalent (jt)) break ;
790 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
791 jt->java_suspend_self();
792 jt->set_suspend_equivalent();
793 }
794 }
795
796 // Try again, but just so we distinguish between futile wakeups and
797 // successful wakeups. The following test isn't algorithmically
798 // necessary, but it helps us maintain sensible statistics.
799 if (TryLock(Self) > 0) break ;
800
801 // The lock is still contested.
802 // Keep a tally of the # of futile wakeups.
803 // Note that the counter is not protected by a lock or updated by atomics.
804 // That is by design - we trade "lossy" counters which are exposed to
805 // races during updates for a lower probe effect.
806 TEVENT (Wait Reentry - futile wakeup) ;
807 ++ nWakeups ;
808
809 // Assuming this is not a spurious wakeup we'll normally
810 // find that _succ == Self.
811 if (_succ == Self) _succ = NULL ;
812
813 // Invariant: after clearing _succ a contending thread
814 // *must* retry _owner before parking.
815 OrderAccess::fence() ;
816
817 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
818 ObjectMonitor::_sync_FutileWakeups->inc() ;
819 }
820 }
821
822 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
823 // Normally we'll find Self on the EntryList.
824 // Unlinking from the EntryList is constant-time and atomic-free.
825 // From the perspective of the lock owner (this thread), the
826 // EntryList is stable and cxq is prepend-only.
827 // The head of cxq is volatile but the interior is stable.
828 // In addition, Self.TState is stable.
829
830 assert (_owner == Self, "invariant") ;
831 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
832 UnlinkAfterAcquire (Self, SelfNode) ;
833 if (_succ == Self) _succ = NULL ;
834 assert (_succ != Self, "invariant") ;
835 SelfNode->TState = ObjectWaiter::TS_RUN ;
836 OrderAccess::fence() ; // see comments at the end of EnterI()
837 }
838
839 // after the thread acquires the lock in ::enter(). Equally, we could defer
840 // unlinking the thread until ::exit()-time.
841
842 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
843 {
844 assert (_owner == Self, "invariant") ;
845 assert (SelfNode->_thread == Self, "invariant") ;
846
847 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
848 // Normal case: remove Self from the DLL EntryList .
849 // This is a constant-time operation.
850 ObjectWaiter * nxt = SelfNode->_next ;
851 ObjectWaiter * prv = SelfNode->_prev ;
852 if (nxt != NULL) nxt->_prev = prv ;
853 if (prv != NULL) prv->_next = nxt ;
854 if (SelfNode == _EntryList ) _EntryList = nxt ;
855 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
856 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
857 TEVENT (Unlink from EntryList) ;
858 } else {
859 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
860 // Inopportune interleaving -- Self is still on the cxq.
861 // This usually means the enqueue of self raced an exiting thread.
862 // Normally we'll find Self near the front of the cxq, so
863 // dequeueing is typically fast. If needbe we can accelerate
864 // this with some MCS/CHL-like bidirectional list hints and advisory
865 // back-links so dequeueing from the interior will normally operate
866 // in constant-time.
867 // Dequeue Self from either the head (with CAS) or from the interior
868 // with a linear-time scan and normal non-atomic memory operations.
869 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
870 // and then unlink Self from EntryList. We have to drain eventually,
871 // so it might as well be now.
872
873 ObjectWaiter * v = _cxq ;
874 assert (v != NULL, "invariant") ;
875 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
876 // The CAS above can fail from interference IFF a "RAT" arrived.
877 // In that case Self must be in the interior and can no longer be
878 // at the head of cxq.
879 if (v == SelfNode) {
880 assert (_cxq != v, "invariant") ;
881 v = _cxq ; // CAS above failed - start scan at head of list
882 }
883 ObjectWaiter * p ;
884 ObjectWaiter * q = NULL ;
885 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
886 q = p ;
887 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
888 }
889 assert (v != SelfNode, "invariant") ;
890 assert (p == SelfNode, "Node not found on cxq") ;
891 assert (p != _cxq, "invariant") ;
892 assert (q != NULL, "invariant") ;
893 assert (q->_next == p, "invariant") ;
894 q->_next = p->_next ;
895 }
896 TEVENT (Unlink from cxq) ;
897 }
898
899 // Diagnostic hygiene ...
900 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
901 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
902 SelfNode->TState = ObjectWaiter::TS_RUN ;
903 }
904
905 // -----------------------------------------------------------------------------
906 // Exit support
907 //
908 // exit()
909 // ~~~~~~
910 // Note that the collector can't reclaim the objectMonitor or deflate
911 // the object out from underneath the thread calling ::exit() as the
912 // thread calling ::exit() never transitions to a stable state.
913 // This inhibits GC, which in turn inhibits asynchronous (and
914 // inopportune) reclamation of "this".
915 //
916 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
917 // There's one exception to the claim above, however. EnterI() can call
918 // exit() to drop a lock if the acquirer has been externally suspended.
919 // In that case exit() is called with _thread_state as _thread_blocked,
920 // but the monitor's _count field is > 0, which inhibits reclamation.
921 //
922 // 1-0 exit
923 // ~~~~~~~~
924 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
925 // the fast-path operators have been optimized so the common ::exit()
926 // operation is 1-0. See i486.ad fast_unlock(), for instance.
927 // The code emitted by fast_unlock() elides the usual MEMBAR. This
928 // greatly improves latency -- MEMBAR and CAS having considerable local
929 // latency on modern processors -- but at the cost of "stranding". Absent the
930 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
931 // ::enter() path, resulting in the entering thread being stranding
932 // and a progress-liveness failure. Stranding is extremely rare.
933 // We use timers (timed park operations) & periodic polling to detect
934 // and recover from stranding. Potentially stranded threads periodically
935 // wake up and poll the lock. See the usage of the _Responsible variable.
936 //
937 // The CAS() in enter provides for safety and exclusion, while the CAS or
938 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
939 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
940 // We detect and recover from stranding with timers.
941 //
942 // If a thread transiently strands it'll park until (a) another
943 // thread acquires the lock and then drops the lock, at which time the
944 // exiting thread will notice and unpark the stranded thread, or, (b)
945 // the timer expires. If the lock is high traffic then the stranding latency
946 // will be low due to (a). If the lock is low traffic then the odds of
947 // stranding are lower, although the worst-case stranding latency
948 // is longer. Critically, we don't want to put excessive load in the
949 // platform's timer subsystem. We want to minimize both the timer injection
950 // rate (timers created/sec) as well as the number of timers active at
951 // any one time. (more precisely, we want to minimize timer-seconds, which is
952 // the integral of the # of active timers at any instant over time).
953 // Both impinge on OS scalability. Given that, at most one thread parked on
954 // a monitor will use a timer.
955
956 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
957 Thread * Self = THREAD ;
958 if (THREAD != _owner) {
959 if (THREAD->is_lock_owned((address) _owner)) {
960 // Transmute _owner from a BasicLock pointer to a Thread address.
961 // We don't need to hold _mutex for this transition.
962 // Non-null to Non-null is safe as long as all readers can
963 // tolerate either flavor.
964 assert (_recursions == 0, "invariant") ;
965 _owner = THREAD ;
966 _recursions = 0 ;
967 OwnerIsThread = 1 ;
968 } else {
969 // NOTE: we need to handle unbalanced monitor enter/exit
970 // in native code by throwing an exception.
971 // TODO: Throw an IllegalMonitorStateException ?
972 TEVENT (Exit - Throw IMSX) ;
973 assert(false, "Non-balanced monitor enter/exit!");
974 if (false) {
975 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
976 }
977 return;
978 }
979 }
980
981 if (_recursions != 0) {
982 _recursions--; // this is simple recursive enter
983 TEVENT (Inflated exit - recursive) ;
984 return ;
985 }
986
987 // Invariant: after setting Responsible=null an thread must execute
988 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
989 if ((SyncFlags & 4) == 0) {
990 _Responsible = NULL ;
991 }
992
993 #if INCLUDE_TRACE
994 // get the owner's thread id for the MonitorEnter event
995 // if it is enabled and the thread isn't suspended
996 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
997 _previous_owner_tid = SharedRuntime::get_java_tid(Self);
998 }
999 #endif
1000
1001 for (;;) {
1002 assert (THREAD == _owner, "invariant") ;
1003
1004
1005 if (Knob_ExitPolicy == 0) {
1006 // release semantics: prior loads and stores from within the critical section
1007 // must not float (reorder) past the following store that drops the lock.
1008 // On SPARC that requires MEMBAR #loadstore|#storestore.
1009 // But of course in TSO #loadstore|#storestore is not required.
1010 // I'd like to write one of the following:
1011 // A. OrderAccess::release() ; _owner = NULL
1012 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
1013 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
1014 // store into a _dummy variable. That store is not needed, but can result
1015 // in massive wasteful coherency traffic on classic SMP systems.
1016 // Instead, I use release_store(), which is implemented as just a simple
1017 // ST on x64, x86 and SPARC.
1018 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1019 OrderAccess::storeload() ; // See if we need to wake a successor
1020 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1021 TEVENT (Inflated exit - simple egress) ;
1022 return ;
1023 }
1024 TEVENT (Inflated exit - complex egress) ;
1025
1026 // Normally the exiting thread is responsible for ensuring succession,
1027 // but if other successors are ready or other entering threads are spinning
1028 // then this thread can simply store NULL into _owner and exit without
1029 // waking a successor. The existence of spinners or ready successors
1030 // guarantees proper succession (liveness). Responsibility passes to the
1031 // ready or running successors. The exiting thread delegates the duty.
1032 // More precisely, if a successor already exists this thread is absolved
1033 // of the responsibility of waking (unparking) one.
1034 //
1035 // The _succ variable is critical to reducing futile wakeup frequency.
1036 // _succ identifies the "heir presumptive" thread that has been made
1037 // ready (unparked) but that has not yet run. We need only one such
1038 // successor thread to guarantee progress.
1039 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1040 // section 3.3 "Futile Wakeup Throttling" for details.
1041 //
1042 // Note that spinners in Enter() also set _succ non-null.
1043 // In the current implementation spinners opportunistically set
1044 // _succ so that exiting threads might avoid waking a successor.
1045 // Another less appealing alternative would be for the exiting thread
1046 // to drop the lock and then spin briefly to see if a spinner managed
1047 // to acquire the lock. If so, the exiting thread could exit
1048 // immediately without waking a successor, otherwise the exiting
1049 // thread would need to dequeue and wake a successor.
1050 // (Note that we'd need to make the post-drop spin short, but no
1051 // shorter than the worst-case round-trip cache-line migration time.
1052 // The dropped lock needs to become visible to the spinner, and then
1053 // the acquisition of the lock by the spinner must become visible to
1054 // the exiting thread).
1055 //
1056
1057 // It appears that an heir-presumptive (successor) must be made ready.
1058 // Only the current lock owner can manipulate the EntryList or
1059 // drain _cxq, so we need to reacquire the lock. If we fail
1060 // to reacquire the lock the responsibility for ensuring succession
1061 // falls to the new owner.
1062 //
1063 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1064 return ;
1065 }
1066 TEVENT (Exit - Reacquired) ;
1067 } else {
1068 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1069 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1070 OrderAccess::storeload() ;
1071 // Ratify the previously observed values.
1072 if (_cxq == NULL || _succ != NULL) {
1073 TEVENT (Inflated exit - simple egress) ;
1074 return ;
1075 }
1076
1077 // inopportune interleaving -- the exiting thread (this thread)
1078 // in the fast-exit path raced an entering thread in the slow-enter
1079 // path.
1080 // We have two choices:
1081 // A. Try to reacquire the lock.
1082 // If the CAS() fails return immediately, otherwise
1083 // we either restart/rerun the exit operation, or simply
1084 // fall-through into the code below which wakes a successor.
1085 // B. If the elements forming the EntryList|cxq are TSM
1086 // we could simply unpark() the lead thread and return
1087 // without having set _succ.
1088 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1089 TEVENT (Inflated exit - reacquired succeeded) ;
1090 return ;
1091 }
1092 TEVENT (Inflated exit - reacquired failed) ;
1093 } else {
1094 TEVENT (Inflated exit - complex egress) ;
1095 }
1096 }
1097
1098 guarantee (_owner == THREAD, "invariant") ;
1099
1100 ObjectWaiter * w = NULL ;
1101 int QMode = Knob_QMode ;
1102
1103 if (QMode == 2 && _cxq != NULL) {
1104 // QMode == 2 : cxq has precedence over EntryList.
1105 // Try to directly wake a successor from the cxq.
1106 // If successful, the successor will need to unlink itself from cxq.
1107 w = _cxq ;
1108 assert (w != NULL, "invariant") ;
1109 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1110 ExitEpilog (Self, w) ;
1111 return ;
1112 }
1113
1114 if (QMode == 3 && _cxq != NULL) {
1115 // Aggressively drain cxq into EntryList at the first opportunity.
1116 // This policy ensure that recently-run threads live at the head of EntryList.
1117 // Drain _cxq into EntryList - bulk transfer.
1118 // First, detach _cxq.
1119 // The following loop is tantamount to: w = swap (&cxq, NULL)
1120 w = _cxq ;
1121 for (;;) {
1122 assert (w != NULL, "Invariant") ;
1123 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1124 if (u == w) break ;
1125 w = u ;
1126 }
1127 assert (w != NULL , "invariant") ;
1128
1129 ObjectWaiter * q = NULL ;
1130 ObjectWaiter * p ;
1131 for (p = w ; p != NULL ; p = p->_next) {
1132 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1133 p->TState = ObjectWaiter::TS_ENTER ;
1134 p->_prev = q ;
1135 q = p ;
1136 }
1137
1138 // Append the RATs to the EntryList
1139 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1140 ObjectWaiter * Tail ;
1141 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1142 if (Tail == NULL) {
1143 _EntryList = w ;
1144 } else {
1145 Tail->_next = w ;
1146 w->_prev = Tail ;
1147 }
1148
1149 // Fall thru into code that tries to wake a successor from EntryList
1150 }
1151
1152 if (QMode == 4 && _cxq != NULL) {
1153 // Aggressively drain cxq into EntryList at the first opportunity.
1154 // This policy ensure that recently-run threads live at the head of EntryList.
1155
1156 // Drain _cxq into EntryList - bulk transfer.
1157 // First, detach _cxq.
1158 // The following loop is tantamount to: w = swap (&cxq, NULL)
1159 w = _cxq ;
1160 for (;;) {
1161 assert (w != NULL, "Invariant") ;
1162 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1163 if (u == w) break ;
1164 w = u ;
1165 }
1166 assert (w != NULL , "invariant") ;
1167
1168 ObjectWaiter * q = NULL ;
1169 ObjectWaiter * p ;
1170 for (p = w ; p != NULL ; p = p->_next) {
1171 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1172 p->TState = ObjectWaiter::TS_ENTER ;
1173 p->_prev = q ;
1174 q = p ;
1175 }
1176
1177 // Prepend the RATs to the EntryList
1178 if (_EntryList != NULL) {
1179 q->_next = _EntryList ;
1180 _EntryList->_prev = q ;
1181 }
1182 _EntryList = w ;
1183
1184 // Fall thru into code that tries to wake a successor from EntryList
1185 }
1186
1187 w = _EntryList ;
1188 if (w != NULL) {
1189 // I'd like to write: guarantee (w->_thread != Self).
1190 // But in practice an exiting thread may find itself on the EntryList.
1191 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1192 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1193 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1194 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1195 // release the lock "O". T2 resumes immediately after the ST of null into
1196 // _owner, above. T2 notices that the EntryList is populated, so it
1197 // reacquires the lock and then finds itself on the EntryList.
1198 // Given all that, we have to tolerate the circumstance where "w" is
1199 // associated with Self.
1200 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1201 ExitEpilog (Self, w) ;
1202 return ;
1203 }
1204
1205 // If we find that both _cxq and EntryList are null then just
1206 // re-run the exit protocol from the top.
1207 w = _cxq ;
1208 if (w == NULL) continue ;
1209
1210 // Drain _cxq into EntryList - bulk transfer.
1211 // First, detach _cxq.
1212 // The following loop is tantamount to: w = swap (&cxq, NULL)
1213 for (;;) {
1214 assert (w != NULL, "Invariant") ;
1215 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1216 if (u == w) break ;
1217 w = u ;
1218 }
1219 TEVENT (Inflated exit - drain cxq into EntryList) ;
1220
1221 assert (w != NULL , "invariant") ;
1222 assert (_EntryList == NULL , "invariant") ;
1223
1224 // Convert the LIFO SLL anchored by _cxq into a DLL.
1225 // The list reorganization step operates in O(LENGTH(w)) time.
1226 // It's critical that this step operate quickly as
1227 // "Self" still holds the outer-lock, restricting parallelism
1228 // and effectively lengthening the critical section.
1229 // Invariant: s chases t chases u.
1230 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1231 // we have faster access to the tail.
1232
1233 if (QMode == 1) {
1234 // QMode == 1 : drain cxq to EntryList, reversing order
1235 // We also reverse the order of the list.
1236 ObjectWaiter * s = NULL ;
1237 ObjectWaiter * t = w ;
1238 ObjectWaiter * u = NULL ;
1239 while (t != NULL) {
1240 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1241 t->TState = ObjectWaiter::TS_ENTER ;
1242 u = t->_next ;
1243 t->_prev = u ;
1244 t->_next = s ;
1245 s = t;
1246 t = u ;
1247 }
1248 _EntryList = s ;
1249 assert (s != NULL, "invariant") ;
1250 } else {
1251 // QMode == 0 or QMode == 2
1252 _EntryList = w ;
1253 ObjectWaiter * q = NULL ;
1254 ObjectWaiter * p ;
1255 for (p = w ; p != NULL ; p = p->_next) {
1256 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1257 p->TState = ObjectWaiter::TS_ENTER ;
1258 p->_prev = q ;
1259 q = p ;
1260 }
1261 }
1262
1263 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1264 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1265
1266 // See if we can abdicate to a spinner instead of waking a thread.
1267 // A primary goal of the implementation is to reduce the
1268 // context-switch rate.
1269 if (_succ != NULL) continue;
1270
1271 w = _EntryList ;
1272 if (w != NULL) {
1273 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1274 ExitEpilog (Self, w) ;
1275 return ;
1276 }
1277 }
1278 }
1279
1280 // ExitSuspendEquivalent:
1281 // A faster alternate to handle_special_suspend_equivalent_condition()
1282 //
1283 // handle_special_suspend_equivalent_condition() unconditionally
1284 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1285 // operations have high latency. Note that in ::enter() we call HSSEC
1286 // while holding the monitor, so we effectively lengthen the critical sections.
1287 //
1288 // There are a number of possible solutions:
1289 //
1290 // A. To ameliorate the problem we might also defer state transitions
1291 // to as late as possible -- just prior to parking.
1292 // Given that, we'd call HSSEC after having returned from park(),
1293 // but before attempting to acquire the monitor. This is only a
1294 // partial solution. It avoids calling HSSEC while holding the
1295 // monitor (good), but it still increases successor reacquisition latency --
1296 // the interval between unparking a successor and the time the successor
1297 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1298 // If we use this technique we can also avoid EnterI()-exit() loop
1299 // in ::enter() where we iteratively drop the lock and then attempt
1300 // to reacquire it after suspending.
1301 //
1302 // B. In the future we might fold all the suspend bits into a
1303 // composite per-thread suspend flag and then update it with CAS().
1304 // Alternately, a Dekker-like mechanism with multiple variables
1305 // would suffice:
1306 // ST Self->_suspend_equivalent = false
1307 // MEMBAR
1308 // LD Self_>_suspend_flags
1309 //
1310
1311
1312 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1313 int Mode = Knob_FastHSSEC ;
1314 if (Mode && !jSelf->is_external_suspend()) {
1315 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1316 jSelf->clear_suspend_equivalent() ;
1317 if (2 == Mode) OrderAccess::storeload() ;
1318 if (!jSelf->is_external_suspend()) return false ;
1319 // We raced a suspension -- fall thru into the slow path
1320 TEVENT (ExitSuspendEquivalent - raced) ;
1321 jSelf->set_suspend_equivalent() ;
1322 }
1323 return jSelf->handle_special_suspend_equivalent_condition() ;
1324 }
1325
1326
1327 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1328 assert (_owner == Self, "invariant") ;
1329
1330 // Exit protocol:
1331 // 1. ST _succ = wakee
1332 // 2. membar #loadstore|#storestore;
1333 // 2. ST _owner = NULL
1334 // 3. unpark(wakee)
1335
1336 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1337 ParkEvent * Trigger = Wakee->_event ;
1338
1339 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1340 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1341 // out-of-scope (non-extant).
1342 Wakee = NULL ;
1343
1344 // Drop the lock
1345 OrderAccess::release_store_ptr (&_owner, NULL) ;
1346 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1347
1348 if (SafepointSynchronize::do_call_back()) {
1349 TEVENT (unpark before SAFEPOINT) ;
1350 }
1351
1352 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1353 Trigger->unpark() ;
1354
1355 // Maintain stats and report events to JVMTI
1356 if (ObjectMonitor::_sync_Parks != NULL) {
1357 ObjectMonitor::_sync_Parks->inc() ;
1358 }
1359 }
1360
1361
1362 // -----------------------------------------------------------------------------
1363 // Class Loader deadlock handling.
1364 //
1365 // complete_exit exits a lock returning recursion count
1366 // complete_exit/reenter operate as a wait without waiting
1367 // complete_exit requires an inflated monitor
1368 // The _owner field is not always the Thread addr even with an
1369 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1370 // thread due to contention.
1371 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1372 Thread * const Self = THREAD;
1373 assert(Self->is_Java_thread(), "Must be Java thread!");
1374 JavaThread *jt = (JavaThread *)THREAD;
1375
1376 DeferredInitialize();
1377
1378 if (THREAD != _owner) {
1379 if (THREAD->is_lock_owned ((address)_owner)) {
1380 assert(_recursions == 0, "internal state error");
1381 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1382 _recursions = 0 ;
1383 OwnerIsThread = 1 ;
1384 }
1385 }
1386
1387 guarantee(Self == _owner, "complete_exit not owner");
1388 intptr_t save = _recursions; // record the old recursion count
1389 _recursions = 0; // set the recursion level to be 0
1390 exit (true, Self) ; // exit the monitor
1391 guarantee (_owner != Self, "invariant");
1392 return save;
1393 }
1394
1395 // reenter() enters a lock and sets recursion count
1396 // complete_exit/reenter operate as a wait without waiting
1397 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1398 Thread * const Self = THREAD;
1399 assert(Self->is_Java_thread(), "Must be Java thread!");
1400 JavaThread *jt = (JavaThread *)THREAD;
1401
1402 guarantee(_owner != Self, "reenter already owner");
1403 enter (THREAD); // enter the monitor
1404 guarantee (_recursions == 0, "reenter recursion");
1405 _recursions = recursions;
1406 return;
1407 }
1408
1409
1410 // -----------------------------------------------------------------------------
1411 // A macro is used below because there may already be a pending
1412 // exception which should not abort the execution of the routines
1413 // which use this (which is why we don't put this into check_slow and
1414 // call it with a CHECK argument).
1415
1416 #define CHECK_OWNER() \
1417 do { \
1418 if (THREAD != _owner) { \
1419 if (THREAD->is_lock_owned((address) _owner)) { \
1420 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1421 _recursions = 0; \
1422 OwnerIsThread = 1 ; \
1423 } else { \
1424 TEVENT (Throw IMSX) ; \
1425 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1426 } \
1427 } \
1428 } while (false)
1429
1430 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1431 // TODO-FIXME: remove check_slow() -- it's likely dead.
1432
1433 void ObjectMonitor::check_slow(TRAPS) {
1434 TEVENT (check_slow - throw IMSX) ;
1435 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1436 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1437 }
1438
1439 static int Adjust (volatile int * adr, int dx) {
1440 int v ;
1441 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1442 return v ;
1443 }
1444
1445 // helper method for posting a monitor wait event
1446 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1447 jlong notifier_tid,
1448 jlong timeout,
1449 bool timedout) {
1450 event->set_klass(((oop)this->object())->klass());
1451 event->set_timeout((TYPE_ULONG)timeout);
1452 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1453 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1454 event->set_timedOut((TYPE_BOOLEAN)timedout);
1455 event->commit();
1456 }
1457
1458 // -----------------------------------------------------------------------------
1459 // Wait/Notify/NotifyAll
1460 //
1461 // Note: a subset of changes to ObjectMonitor::wait()
1462 // will need to be replicated in complete_exit above
1463 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1464 Thread * const Self = THREAD ;
1465 assert(Self->is_Java_thread(), "Must be Java thread!");
1466 JavaThread *jt = (JavaThread *)THREAD;
1467
1468 DeferredInitialize () ;
1469
1470 // Throw IMSX or IEX.
1471 CHECK_OWNER();
1472
1473 EventJavaMonitorWait event;
1474
1475 // check for a pending interrupt
1476 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1477 // post monitor waited event. Note that this is past-tense, we are done waiting.
1478 if (JvmtiExport::should_post_monitor_waited()) {
1479 // Note: 'false' parameter is passed here because the
1480 // wait was not timed out due to thread interrupt.
1481 JvmtiExport::post_monitor_waited(jt, this, false);
1482
1483 // In this short circuit of the monitor wait protocol, the
1484 // current thread never drops ownership of the monitor and
1485 // never gets added to the wait queue so the current thread
1486 // cannot be made the successor. This means that the
1487 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1488 // consume an unpark() meant for the ParkEvent associated with
1489 // this ObjectMonitor.
1490 }
1491 if (event.should_commit()) {
1492 post_monitor_wait_event(&event, 0, millis, false);
1493 }
1494 TEVENT (Wait - Throw IEX) ;
1495 THROW(vmSymbols::java_lang_InterruptedException());
1496 return ;
1497 }
1498
1499 TEVENT (Wait) ;
1500
1501 assert (Self->_Stalled == 0, "invariant") ;
1502 Self->_Stalled = intptr_t(this) ;
1503 jt->set_current_waiting_monitor(this);
1504
1505 // create a node to be put into the queue
1506 // Critically, after we reset() the event but prior to park(), we must check
1507 // for a pending interrupt.
1508 ObjectWaiter node(Self);
1509 node.TState = ObjectWaiter::TS_WAIT ;
1510 Self->_ParkEvent->reset() ;
1511 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1512
1513 // Enter the waiting queue, which is a circular doubly linked list in this case
1514 // but it could be a priority queue or any data structure.
1515 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1516 // by the the owner of the monitor *except* in the case where park()
1517 // returns because of a timeout of interrupt. Contention is exceptionally rare
1518 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1519
1520 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1521 AddWaiter (&node) ;
1522 Thread::SpinRelease (&_WaitSetLock) ;
1523
1524 if ((SyncFlags & 4) == 0) {
1525 _Responsible = NULL ;
1526 }
1527 intptr_t save = _recursions; // record the old recursion count
1528 _waiters++; // increment the number of waiters
1529 _recursions = 0; // set the recursion level to be 1
1530 exit (true, Self) ; // exit the monitor
1531 guarantee (_owner != Self, "invariant") ;
1532
1533 // The thread is on the WaitSet list - now park() it.
1534 // On MP systems it's conceivable that a brief spin before we park
1535 // could be profitable.
1536 //
1537 // TODO-FIXME: change the following logic to a loop of the form
1538 // while (!timeout && !interrupted && _notified == 0) park()
1539
1540 int ret = OS_OK ;
1541 int WasNotified = 0 ;
1542 { // State transition wrappers
1543 OSThread* osthread = Self->osthread();
1544 OSThreadWaitState osts(osthread, true);
1545 {
1546 ThreadBlockInVM tbivm(jt);
1547 // Thread is in thread_blocked state and oop access is unsafe.
1548 jt->set_suspend_equivalent();
1549
1550 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1551 // Intentionally empty
1552 } else
1553 if (node._notified == 0) {
1554 if (millis <= 0) {
1555 Self->_ParkEvent->park () ;
1556 } else {
1557 ret = Self->_ParkEvent->park (millis) ;
1558 }
1559 }
1560
1561 // were we externally suspended while we were waiting?
1562 if (ExitSuspendEquivalent (jt)) {
1563 // TODO-FIXME: add -- if succ == Self then succ = null.
1564 jt->java_suspend_self();
1565 }
1566
1567 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1568
1569
1570 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1571 // from the WaitSet to the EntryList.
1572 // See if we need to remove Node from the WaitSet.
1573 // We use double-checked locking to avoid grabbing _WaitSetLock
1574 // if the thread is not on the wait queue.
1575 //
1576 // Note that we don't need a fence before the fetch of TState.
1577 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1578 // written by the is thread. (perhaps the fetch might even be satisfied
1579 // by a look-aside into the processor's own store buffer, although given
1580 // the length of the code path between the prior ST and this load that's
1581 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1582 // then we'll acquire the lock and then re-fetch a fresh TState value.
1583 // That is, we fail toward safety.
1584
1585 if (node.TState == ObjectWaiter::TS_WAIT) {
1586 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1587 if (node.TState == ObjectWaiter::TS_WAIT) {
1588 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1589 assert(node._notified == 0, "invariant");
1590 node.TState = ObjectWaiter::TS_RUN ;
1591 }
1592 Thread::SpinRelease (&_WaitSetLock) ;
1593 }
1594
1595 // The thread is now either on off-list (TS_RUN),
1596 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1597 // The Node's TState variable is stable from the perspective of this thread.
1598 // No other threads will asynchronously modify TState.
1599 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1600 OrderAccess::loadload() ;
1601 if (_succ == Self) _succ = NULL ;
1602 WasNotified = node._notified ;
1603
1604 // Reentry phase -- reacquire the monitor.
1605 // re-enter contended monitor after object.wait().
1606 // retain OBJECT_WAIT state until re-enter successfully completes
1607 // Thread state is thread_in_vm and oop access is again safe,
1608 // although the raw address of the object may have changed.
1609 // (Don't cache naked oops over safepoints, of course).
1610
1611 // post monitor waited event. Note that this is past-tense, we are done waiting.
1612 if (JvmtiExport::should_post_monitor_waited()) {
1613 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1614
1615 if (node._notified != 0 && _succ == Self) {
1616 // In this part of the monitor wait-notify-reenter protocol it
1617 // is possible (and normal) for another thread to do a fastpath
1618 // monitor enter-exit while this thread is still trying to get
1619 // to the reenter portion of the protocol.
1620 //
1621 // The ObjectMonitor was notified and the current thread is
1622 // the successor which also means that an unpark() has already
1623 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1624 // consume the unpark() that was done when the successor was
1625 // set because the same ParkEvent is shared between Java
1626 // monitors and JVM/TI RawMonitors (for now).
1627 //
1628 // We redo the unpark() to ensure forward progress, i.e., we
1629 // don't want all pending threads hanging (parked) with none
1630 // entering the unlocked monitor.
1631 node._event->unpark();
1632 }
1633 }
1634
1635 if (event.should_commit()) {
1636 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1637 }
1638
1639 OrderAccess::fence() ;
1640
1641 assert (Self->_Stalled != 0, "invariant") ;
1642 Self->_Stalled = 0 ;
1643
1644 assert (_owner != Self, "invariant") ;
1645 ObjectWaiter::TStates v = node.TState ;
1646 if (v == ObjectWaiter::TS_RUN) {
1647 enter (Self) ;
1648 } else {
1649 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1650 ReenterI (Self, &node) ;
1651 node.wait_reenter_end(this);
1652 }
1653
1654 // Self has reacquired the lock.
1655 // Lifecycle - the node representing Self must not appear on any queues.
1656 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1657 // want residual elements associated with this thread left on any lists.
1658 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1659 assert (_owner == Self, "invariant") ;
1660 assert (_succ != Self , "invariant") ;
1661 } // OSThreadWaitState()
1662
1663 jt->set_current_waiting_monitor(NULL);
1664
1665 guarantee (_recursions == 0, "invariant") ;
1666 _recursions = save; // restore the old recursion count
1667 _waiters--; // decrement the number of waiters
1668
1669 // Verify a few postconditions
1670 assert (_owner == Self , "invariant") ;
1671 assert (_succ != Self , "invariant") ;
1672 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1673
1674 if (SyncFlags & 32) {
1675 OrderAccess::fence() ;
1676 }
1677
1678 // check if the notification happened
1679 if (!WasNotified) {
1680 // no, it could be timeout or Thread.interrupt() or both
1681 // check for interrupt event, otherwise it is timeout
1682 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1683 TEVENT (Wait - throw IEX from epilog) ;
1684 THROW(vmSymbols::java_lang_InterruptedException());
1685 }
1686 }
1687
1688 // NOTE: Spurious wake up will be consider as timeout.
1689 // Monitor notify has precedence over thread interrupt.
1690 }
1691
1692
1693 // Consider:
1694 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1695 // then instead of transferring a thread from the WaitSet to the EntryList
1696 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1697
1698 void ObjectMonitor::notify(TRAPS) {
1699 CHECK_OWNER();
1700 if (_WaitSet == NULL) {
1701 TEVENT (Empty-Notify) ;
1702 return ;
1703 }
1704 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1705
1706 int Policy = Knob_MoveNotifyee ;
1707
1708 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1709 ObjectWaiter * iterator = DequeueWaiter() ;
1710 if (iterator != NULL) {
1711 TEVENT (Notify1 - Transfer) ;
1712 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1713 guarantee (iterator->_notified == 0, "invariant") ;
1714 if (Policy != 4) {
1715 iterator->TState = ObjectWaiter::TS_ENTER ;
1716 }
1717 iterator->_notified = 1 ;
1718 Thread * Self = THREAD;
1719 iterator->_notifier_tid = Self->osthread()->thread_id();
1720
1721 ObjectWaiter * List = _EntryList ;
1722 if (List != NULL) {
1723 assert (List->_prev == NULL, "invariant") ;
1724 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1725 assert (List != iterator, "invariant") ;
1726 }
1727
1728 if (Policy == 0) { // prepend to EntryList
1729 if (List == NULL) {
1730 iterator->_next = iterator->_prev = NULL ;
1731 _EntryList = iterator ;
1732 } else {
1733 List->_prev = iterator ;
1734 iterator->_next = List ;
1735 iterator->_prev = NULL ;
1736 _EntryList = iterator ;
1737 }
1738 } else
1739 if (Policy == 1) { // append to EntryList
1740 if (List == NULL) {
1741 iterator->_next = iterator->_prev = NULL ;
1742 _EntryList = iterator ;
1743 } else {
1744 // CONSIDER: finding the tail currently requires a linear-time walk of
1745 // the EntryList. We can make tail access constant-time by converting to
1746 // a CDLL instead of using our current DLL.
1747 ObjectWaiter * Tail ;
1748 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1749 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1750 Tail->_next = iterator ;
1751 iterator->_prev = Tail ;
1752 iterator->_next = NULL ;
1753 }
1754 } else
1755 if (Policy == 2) { // prepend to cxq
1756 // prepend to cxq
1757 if (List == NULL) {
1758 iterator->_next = iterator->_prev = NULL ;
1759 _EntryList = iterator ;
1760 } else {
1761 iterator->TState = ObjectWaiter::TS_CXQ ;
1762 for (;;) {
1763 ObjectWaiter * Front = _cxq ;
1764 iterator->_next = Front ;
1765 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1766 break ;
1767 }
1768 }
1769 }
1770 } else
1771 if (Policy == 3) { // append to cxq
1772 iterator->TState = ObjectWaiter::TS_CXQ ;
1773 for (;;) {
1774 ObjectWaiter * Tail ;
1775 Tail = _cxq ;
1776 if (Tail == NULL) {
1777 iterator->_next = NULL ;
1778 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1779 break ;
1780 }
1781 } else {
1782 while (Tail->_next != NULL) Tail = Tail->_next ;
1783 Tail->_next = iterator ;
1784 iterator->_prev = Tail ;
1785 iterator->_next = NULL ;
1786 break ;
1787 }
1788 }
1789 } else {
1790 ParkEvent * ev = iterator->_event ;
1791 iterator->TState = ObjectWaiter::TS_RUN ;
1792 OrderAccess::fence() ;
1793 ev->unpark() ;
1794 }
1795
1796 if (Policy < 4) {
1797 iterator->wait_reenter_begin(this);
1798 }
1799
1800 // _WaitSetLock protects the wait queue, not the EntryList. We could
1801 // move the add-to-EntryList operation, above, outside the critical section
1802 // protected by _WaitSetLock. In practice that's not useful. With the
1803 // exception of wait() timeouts and interrupts the monitor owner
1804 // is the only thread that grabs _WaitSetLock. There's almost no contention
1805 // on _WaitSetLock so it's not profitable to reduce the length of the
1806 // critical section.
1807 }
1808
1809 Thread::SpinRelease (&_WaitSetLock) ;
1810
1811 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1812 ObjectMonitor::_sync_Notifications->inc() ;
1813 }
1814 }
1815
1816
1817 void ObjectMonitor::notifyAll(TRAPS) {
1818 CHECK_OWNER();
1819 ObjectWaiter* iterator;
1820 if (_WaitSet == NULL) {
1821 TEVENT (Empty-NotifyAll) ;
1822 return ;
1823 }
1824 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1825
1826 int Policy = Knob_MoveNotifyee ;
1827 int Tally = 0 ;
1828 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1829
1830 for (;;) {
1831 iterator = DequeueWaiter () ;
1832 if (iterator == NULL) break ;
1833 TEVENT (NotifyAll - Transfer1) ;
1834 ++Tally ;
1835
1836 // Disposition - what might we do with iterator ?
1837 // a. add it directly to the EntryList - either tail or head.
1838 // b. push it onto the front of the _cxq.
1839 // For now we use (a).
1840
1841 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1842 guarantee (iterator->_notified == 0, "invariant") ;
1843 iterator->_notified = 1 ;
1844 Thread * Self = THREAD;
1845 iterator->_notifier_tid = Self->osthread()->thread_id();
1846 if (Policy != 4) {
1847 iterator->TState = ObjectWaiter::TS_ENTER ;
1848 }
1849
1850 ObjectWaiter * List = _EntryList ;
1851 if (List != NULL) {
1852 assert (List->_prev == NULL, "invariant") ;
1853 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1854 assert (List != iterator, "invariant") ;
1855 }
1856
1857 if (Policy == 0) { // prepend to EntryList
1858 if (List == NULL) {
1859 iterator->_next = iterator->_prev = NULL ;
1860 _EntryList = iterator ;
1861 } else {
1862 List->_prev = iterator ;
1863 iterator->_next = List ;
1864 iterator->_prev = NULL ;
1865 _EntryList = iterator ;
1866 }
1867 } else
1868 if (Policy == 1) { // append to EntryList
1869 if (List == NULL) {
1870 iterator->_next = iterator->_prev = NULL ;
1871 _EntryList = iterator ;
1872 } else {
1873 // CONSIDER: finding the tail currently requires a linear-time walk of
1874 // the EntryList. We can make tail access constant-time by converting to
1875 // a CDLL instead of using our current DLL.
1876 ObjectWaiter * Tail ;
1877 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1878 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1879 Tail->_next = iterator ;
1880 iterator->_prev = Tail ;
1881 iterator->_next = NULL ;
1882 }
1883 } else
1884 if (Policy == 2) { // prepend to cxq
1885 // prepend to cxq
1886 iterator->TState = ObjectWaiter::TS_CXQ ;
1887 for (;;) {
1888 ObjectWaiter * Front = _cxq ;
1889 iterator->_next = Front ;
1890 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1891 break ;
1892 }
1893 }
1894 } else
1895 if (Policy == 3) { // append to cxq
1896 iterator->TState = ObjectWaiter::TS_CXQ ;
1897 for (;;) {
1898 ObjectWaiter * Tail ;
1899 Tail = _cxq ;
1900 if (Tail == NULL) {
1901 iterator->_next = NULL ;
1902 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1903 break ;
1904 }
1905 } else {
1906 while (Tail->_next != NULL) Tail = Tail->_next ;
1907 Tail->_next = iterator ;
1908 iterator->_prev = Tail ;
1909 iterator->_next = NULL ;
1910 break ;
1911 }
1912 }
1913 } else {
1914 ParkEvent * ev = iterator->_event ;
1915 iterator->TState = ObjectWaiter::TS_RUN ;
1916 OrderAccess::fence() ;
1917 ev->unpark() ;
1918 }
1919
1920 if (Policy < 4) {
1921 iterator->wait_reenter_begin(this);
1922 }
1923
1924 // _WaitSetLock protects the wait queue, not the EntryList. We could
1925 // move the add-to-EntryList operation, above, outside the critical section
1926 // protected by _WaitSetLock. In practice that's not useful. With the
1927 // exception of wait() timeouts and interrupts the monitor owner
1928 // is the only thread that grabs _WaitSetLock. There's almost no contention
1929 // on _WaitSetLock so it's not profitable to reduce the length of the
1930 // critical section.
1931 }
1932
1933 Thread::SpinRelease (&_WaitSetLock) ;
1934
1935 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1936 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1937 }
1938 }
1939
1940 // -----------------------------------------------------------------------------
1941 // Adaptive Spinning Support
1942 //
1943 // Adaptive spin-then-block - rational spinning
1944 //
1945 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1946 // algorithm. On high order SMP systems it would be better to start with
1947 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1948 // a contending thread could enqueue itself on the cxq and then spin locally
1949 // on a thread-specific variable such as its ParkEvent._Event flag.
1950 // That's left as an exercise for the reader. Note that global spinning is
1951 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1952 // low latency and massive bandwidth.
1953 //
1954 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1955 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1956 // (duration) or we can fix the count at approximately the duration of
1957 // a context switch and vary the frequency. Of course we could also
1958 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1959 // For a description of 'Adaptive spin-then-block mutual exclusion in
1960 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1961 //
1962 // This implementation varies the duration "D", where D varies with
1963 // the success rate of recent spin attempts. (D is capped at approximately
1964 // length of a round-trip context switch). The success rate for recent
1965 // spin attempts is a good predictor of the success rate of future spin
1966 // attempts. The mechanism adapts automatically to varying critical
1967 // section length (lock modality), system load and degree of parallelism.
1968 // D is maintained per-monitor in _SpinDuration and is initialized
1969 // optimistically. Spin frequency is fixed at 100%.
1970 //
1971 // Note that _SpinDuration is volatile, but we update it without locks
1972 // or atomics. The code is designed so that _SpinDuration stays within
1973 // a reasonable range even in the presence of races. The arithmetic
1974 // operations on _SpinDuration are closed over the domain of legal values,
1975 // so at worst a race will install and older but still legal value.
1976 // At the very worst this introduces some apparent non-determinism.
1977 // We might spin when we shouldn't or vice-versa, but since the spin
1978 // count are relatively short, even in the worst case, the effect is harmless.
1979 //
1980 // Care must be taken that a low "D" value does not become an
1981 // an absorbing state. Transient spinning failures -- when spinning
1982 // is overall profitable -- should not cause the system to converge
1983 // on low "D" values. We want spinning to be stable and predictable
1984 // and fairly responsive to change and at the same time we don't want
1985 // it to oscillate, become metastable, be "too" non-deterministic,
1986 // or converge on or enter undesirable stable absorbing states.
1987 //
1988 // We implement a feedback-based control system -- using past behavior
1989 // to predict future behavior. We face two issues: (a) if the
1990 // input signal is random then the spin predictor won't provide optimal
1991 // results, and (b) if the signal frequency is too high then the control
1992 // system, which has some natural response lag, will "chase" the signal.
1993 // (b) can arise from multimodal lock hold times. Transient preemption
1994 // can also result in apparent bimodal lock hold times.
1995 // Although sub-optimal, neither condition is particularly harmful, as
1996 // in the worst-case we'll spin when we shouldn't or vice-versa.
1997 // The maximum spin duration is rather short so the failure modes aren't bad.
1998 // To be conservative, I've tuned the gain in system to bias toward
1999 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2000 // "rings" or oscillates between spinning and not spinning. This happens
2001 // when spinning is just on the cusp of profitability, however, so the
2002 // situation is not dire. The state is benign -- there's no need to add
2003 // hysteresis control to damp the transition rate between spinning and
2004 // not spinning.
2005 //
2006
2007 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
2008 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
2009
2010 // Spinning: Fixed frequency (100%), vary duration
2011
2012
2013 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
2014
2015 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2016 int ctr = Knob_FixedSpin ;
2017 if (ctr != 0) {
2018 while (--ctr >= 0) {
2019 if (TryLock (Self) > 0) return 1 ;
2020 SpinPause () ;
2021 }
2022 return 0 ;
2023 }
2024
2025 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
2026 if (TryLock(Self) > 0) {
2027 // Increase _SpinDuration ...
2028 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2029 // Raising _SpurDuration to the poverty line is key.
2030 int x = _SpinDuration ;
2031 if (x < Knob_SpinLimit) {
2032 if (x < Knob_Poverty) x = Knob_Poverty ;
2033 _SpinDuration = x + Knob_BonusB ;
2034 }
2035 return 1 ;
2036 }
2037 SpinPause () ;
2038 }
2039
2040 // Admission control - verify preconditions for spinning
2041 //
2042 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2043 // becoming an absorbing state. Put another way, we spin briefly to
2044 // sample, just in case the system load, parallelism, contention, or lock
2045 // modality changed.
2046 //
2047 // Consider the following alternative:
2048 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2049 // spin attempt. "Periodically" might mean after a tally of
2050 // the # of failed spin attempts (or iterations) reaches some threshold.
2051 // This takes us into the realm of 1-out-of-N spinning, where we
2052 // hold the duration constant but vary the frequency.
2053
2054 ctr = _SpinDuration ;
2055 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2056 if (ctr <= 0) return 0 ;
2057
2058 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2059 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2060 TEVENT (Spin abort - notrunnable [TOP]);
2061 return 0 ;
2062 }
2063
2064 int MaxSpin = Knob_MaxSpinners ;
2065 if (MaxSpin >= 0) {
2066 if (_Spinner > MaxSpin) {
2067 TEVENT (Spin abort -- too many spinners) ;
2068 return 0 ;
2069 }
2070 // Slighty racy, but benign ...
2071 Adjust (&_Spinner, 1) ;
2072 }
2073
2074 // We're good to spin ... spin ingress.
2075 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2076 // when preparing to LD...CAS _owner, etc and the CAS is likely
2077 // to succeed.
2078 int hits = 0 ;
2079 int msk = 0 ;
2080 int caspty = Knob_CASPenalty ;
2081 int oxpty = Knob_OXPenalty ;
2082 int sss = Knob_SpinSetSucc ;
2083 if (sss && _succ == NULL ) _succ = Self ;
2084 Thread * prv = NULL ;
2085
2086 // There are three ways to exit the following loop:
2087 // 1. A successful spin where this thread has acquired the lock.
2088 // 2. Spin failure with prejudice
2089 // 3. Spin failure without prejudice
2090
2091 while (--ctr >= 0) {
2092
2093 // Periodic polling -- Check for pending GC
2094 // Threads may spin while they're unsafe.
2095 // We don't want spinning threads to delay the JVM from reaching
2096 // a stop-the-world safepoint or to steal cycles from GC.
2097 // If we detect a pending safepoint we abort in order that
2098 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2099 // this thread, if safe, doesn't steal cycles from GC.
2100 // This is in keeping with the "no loitering in runtime" rule.
2101 // We periodically check to see if there's a safepoint pending.
2102 if ((ctr & 0xFF) == 0) {
2103 if (SafepointSynchronize::do_call_back()) {
2104 TEVENT (Spin: safepoint) ;
2105 goto Abort ; // abrupt spin egress
2106 }
2107 if (Knob_UsePause & 1) SpinPause () ;
2108
2109 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2110 if (hits > 50 && scb != NULL) {
2111 int abend = (*scb)(SpinCallbackArgument, 0) ;
2112 }
2113 }
2114
2115 if (Knob_UsePause & 2) SpinPause() ;
2116
2117 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2118 // This is useful on classic SMP systems, but is of less utility on
2119 // N1-style CMT platforms.
2120 //
2121 // Trade-off: lock acquisition latency vs coherency bandwidth.
2122 // Lock hold times are typically short. A histogram
2123 // of successful spin attempts shows that we usually acquire
2124 // the lock early in the spin. That suggests we want to
2125 // sample _owner frequently in the early phase of the spin,
2126 // but then back-off and sample less frequently as the spin
2127 // progresses. The back-off makes a good citizen on SMP big
2128 // SMP systems. Oversampling _owner can consume excessive
2129 // coherency bandwidth. Relatedly, if we _oversample _owner we
2130 // can inadvertently interfere with the the ST m->owner=null.
2131 // executed by the lock owner.
2132 if (ctr & msk) continue ;
2133 ++hits ;
2134 if ((hits & 0xF) == 0) {
2135 // The 0xF, above, corresponds to the exponent.
2136 // Consider: (msk+1)|msk
2137 msk = ((msk << 2)|3) & BackOffMask ;
2138 }
2139
2140 // Probe _owner with TATAS
2141 // If this thread observes the monitor transition or flicker
2142 // from locked to unlocked to locked, then the odds that this
2143 // thread will acquire the lock in this spin attempt go down
2144 // considerably. The same argument applies if the CAS fails
2145 // or if we observe _owner change from one non-null value to
2146 // another non-null value. In such cases we might abort
2147 // the spin without prejudice or apply a "penalty" to the
2148 // spin count-down variable "ctr", reducing it by 100, say.
2149
2150 Thread * ox = (Thread *) _owner ;
2151 if (ox == NULL) {
2152 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2153 if (ox == NULL) {
2154 // The CAS succeeded -- this thread acquired ownership
2155 // Take care of some bookkeeping to exit spin state.
2156 if (sss && _succ == Self) {
2157 _succ = NULL ;
2158 }
2159 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2160
2161 // Increase _SpinDuration :
2162 // The spin was successful (profitable) so we tend toward
2163 // longer spin attempts in the future.
2164 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2165 // If we acquired the lock early in the spin cycle it
2166 // makes sense to increase _SpinDuration proportionally.
2167 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2168 int x = _SpinDuration ;
2169 if (x < Knob_SpinLimit) {
2170 if (x < Knob_Poverty) x = Knob_Poverty ;
2171 _SpinDuration = x + Knob_Bonus ;
2172 }
2173 return 1 ;
2174 }
2175
2176 // The CAS failed ... we can take any of the following actions:
2177 // * penalize: ctr -= Knob_CASPenalty
2178 // * exit spin with prejudice -- goto Abort;
2179 // * exit spin without prejudice.
2180 // * Since CAS is high-latency, retry again immediately.
2181 prv = ox ;
2182 TEVENT (Spin: cas failed) ;
2183 if (caspty == -2) break ;
2184 if (caspty == -1) goto Abort ;
2185 ctr -= caspty ;
2186 continue ;
2187 }
2188
2189 // Did lock ownership change hands ?
2190 if (ox != prv && prv != NULL ) {
2191 TEVENT (spin: Owner changed)
2192 if (oxpty == -2) break ;
2193 if (oxpty == -1) goto Abort ;
2194 ctr -= oxpty ;
2195 }
2196 prv = ox ;
2197
2198 // Abort the spin if the owner is not executing.
2199 // The owner must be executing in order to drop the lock.
2200 // Spinning while the owner is OFFPROC is idiocy.
2201 // Consider: ctr -= RunnablePenalty ;
2202 if (Knob_OState && NotRunnable (Self, ox)) {
2203 TEVENT (Spin abort - notrunnable);
2204 goto Abort ;
2205 }
2206 if (sss && _succ == NULL ) _succ = Self ;
2207 }
2208
2209 // Spin failed with prejudice -- reduce _SpinDuration.
2210 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2211 // AIMD is globally stable.
2212 TEVENT (Spin failure) ;
2213 {
2214 int x = _SpinDuration ;
2215 if (x > 0) {
2216 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2217 // This is globally sample and tends to damp the response.
2218 x -= Knob_Penalty ;
2219 if (x < 0) x = 0 ;
2220 _SpinDuration = x ;
2221 }
2222 }
2223
2224 Abort:
2225 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2226 if (sss && _succ == Self) {
2227 _succ = NULL ;
2228 // Invariant: after setting succ=null a contending thread
2229 // must recheck-retry _owner before parking. This usually happens
2230 // in the normal usage of TrySpin(), but it's safest
2231 // to make TrySpin() as foolproof as possible.
2232 OrderAccess::fence() ;
2233 if (TryLock(Self) > 0) return 1 ;
2234 }
2235 return 0 ;
2236 }
2237
2238 // NotRunnable() -- informed spinning
2239 //
2240 // Don't bother spinning if the owner is not eligible to drop the lock.
2241 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2242 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2243 // The thread must be runnable in order to drop the lock in timely fashion.
2244 // If the _owner is not runnable then spinning will not likely be
2245 // successful (profitable).
2246 //
2247 // Beware -- the thread referenced by _owner could have died
2248 // so a simply fetch from _owner->_thread_state might trap.
2249 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2250 // Because of the lifecycle issues the schedctl and _thread_state values
2251 // observed by NotRunnable() might be garbage. NotRunnable must
2252 // tolerate this and consider the observed _thread_state value
2253 // as advisory.
2254 //
2255 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2256 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2257 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2258 // with the LSB of _owner. Another option would be to probablistically probe
2259 // the putative _owner->TypeTag value.
2260 //
2261 // Checking _thread_state isn't perfect. Even if the thread is
2262 // in_java it might be blocked on a page-fault or have been preempted
2263 // and sitting on a ready/dispatch queue. _thread state in conjunction
2264 // with schedctl.sc_state gives us a good picture of what the
2265 // thread is doing, however.
2266 //
2267 // TODO: check schedctl.sc_state.
2268 // We'll need to use SafeFetch32() to read from the schedctl block.
2269 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2270 //
2271 // The return value from NotRunnable() is *advisory* -- the
2272 // result is based on sampling and is not necessarily coherent.
2273 // The caller must tolerate false-negative and false-positive errors.
2274 // Spinning, in general, is probabilistic anyway.
2275
2276
2277 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2278 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2279 if (!OwnerIsThread) return 0 ;
2280
2281 if (ox == NULL) return 0 ;
2282
2283 // Avoid transitive spinning ...
2284 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2285 // Immediately after T1 acquires L it's possible that T2, also
2286 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2287 // This occurs transiently after T1 acquired L but before
2288 // T1 managed to clear T1.Stalled. T2 does not need to abort
2289 // its spin in this circumstance.
2290 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2291
2292 if (BlockedOn == 1) return 1 ;
2293 if (BlockedOn != 0) {
2294 return BlockedOn != intptr_t(this) && _owner == ox ;
2295 }
2296
2297 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2298 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2299 // consider also: jst != _thread_in_Java -- but that's overspecific.
2300 return jst == _thread_blocked || jst == _thread_in_native ;
2301 }
2302
2303
2304 // -----------------------------------------------------------------------------
2305 // WaitSet management ...
2306
2307 ObjectWaiter::ObjectWaiter(Thread* thread) {
2308 _next = NULL;
2309 _prev = NULL;
2310 _notified = 0;
2311 TState = TS_RUN ;
2312 _thread = thread;
2313 _event = thread->_ParkEvent ;
2314 _active = false;
2315 assert (_event != NULL, "invariant") ;
2316 }
2317
2318 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2319 JavaThread *jt = (JavaThread *)this->_thread;
2320 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2321 }
2322
2323 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2324 JavaThread *jt = (JavaThread *)this->_thread;
2325 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2326 }
2327
2328 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2329 assert(node != NULL, "should not dequeue NULL node");
2330 assert(node->_prev == NULL, "node already in list");
2331 assert(node->_next == NULL, "node already in list");
2332 // put node at end of queue (circular doubly linked list)
2333 if (_WaitSet == NULL) {
2334 _WaitSet = node;
2335 node->_prev = node;
2336 node->_next = node;
2337 } else {
2338 ObjectWaiter* head = _WaitSet ;
2339 ObjectWaiter* tail = head->_prev;
2340 assert(tail->_next == head, "invariant check");
2341 tail->_next = node;
2342 head->_prev = node;
2343 node->_next = head;
2344 node->_prev = tail;
2345 }
2346 }
2347
2348 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2349 // dequeue the very first waiter
2350 ObjectWaiter* waiter = _WaitSet;
2351 if (waiter) {
2352 DequeueSpecificWaiter(waiter);
2353 }
2354 return waiter;
2355 }
2356
2357 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2358 assert(node != NULL, "should not dequeue NULL node");
2359 assert(node->_prev != NULL, "node already removed from list");
2360 assert(node->_next != NULL, "node already removed from list");
2361 // when the waiter has woken up because of interrupt,
2362 // timeout or other spurious wake-up, dequeue the
2363 // waiter from waiting list
2364 ObjectWaiter* next = node->_next;
2365 if (next == node) {
2366 assert(node->_prev == node, "invariant check");
2367 _WaitSet = NULL;
2368 } else {
2369 ObjectWaiter* prev = node->_prev;
2370 assert(prev->_next == node, "invariant check");
2371 assert(next->_prev == node, "invariant check");
2372 next->_prev = prev;
2373 prev->_next = next;
2374 if (_WaitSet == node) {
2375 _WaitSet = next;
2376 }
2377 }
2378 node->_next = NULL;
2379 node->_prev = NULL;
2380 }
2381
2382 // -----------------------------------------------------------------------------
2383 // PerfData support
2384 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2385 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2386 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2387 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2388 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2389 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2390 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2391 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2392 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2393 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2394 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2395 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2396 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2397 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2398 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2399 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2400 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2401 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2402
2403 // One-shot global initialization for the sync subsystem.
2404 // We could also defer initialization and initialize on-demand
2405 // the first time we call inflate(). Initialization would
2406 // be protected - like so many things - by the MonitorCache_lock.
2407
2408 void ObjectMonitor::Initialize () {
2409 static int InitializationCompleted = 0 ;
2410 assert (InitializationCompleted == 0, "invariant") ;
2411 InitializationCompleted = 1 ;
2412 if (UsePerfData) {
2413 EXCEPTION_MARK ;
2414 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2415 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2416 NEWPERFCOUNTER(_sync_Inflations) ;
2417 NEWPERFCOUNTER(_sync_Deflations) ;
2418 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2419 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2420 NEWPERFCOUNTER(_sync_Parks) ;
2421 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2422 NEWPERFCOUNTER(_sync_Notifications) ;
2423 NEWPERFCOUNTER(_sync_SlowEnter) ;
2424 NEWPERFCOUNTER(_sync_SlowExit) ;
2425 NEWPERFCOUNTER(_sync_SlowNotify) ;
2426 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2427 NEWPERFCOUNTER(_sync_FailedSpins) ;
2428 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2429 NEWPERFCOUNTER(_sync_PrivateA) ;
2430 NEWPERFCOUNTER(_sync_PrivateB) ;
2431 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2432 NEWPERFCOUNTER(_sync_MonScavenged) ;
2433 NEWPERFVARIABLE(_sync_MonExtant) ;
2434 #undef NEWPERFCOUNTER
2435 }
2436 }
2437
2438
2439 // Compile-time asserts
2440 // When possible, it's better to catch errors deterministically at
2441 // compile-time than at runtime. The down-side to using compile-time
2442 // asserts is that error message -- often something about negative array
2443 // indices -- is opaque.
2444
2445 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2446
2447 void ObjectMonitor::ctAsserts() {
2448 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2449 }
2450
2451
2452 static char * kvGet (char * kvList, const char * Key) {
2453 if (kvList == NULL) return NULL ;
2454 size_t n = strlen (Key) ;
2455 char * Search ;
2456 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2457 if (strncmp (Search, Key, n) == 0) {
2458 if (Search[n] == '=') return Search + n + 1 ;
2459 if (Search[n] == 0) return (char *) "1" ;
2460 }
2461 }
2462 return NULL ;
2463 }
2464
2465 static int kvGetInt (char * kvList, const char * Key, int Default) {
2466 char * v = kvGet (kvList, Key) ;
2467 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2468 if (Knob_ReportSettings && v != NULL) {
2469 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2470 ::fflush (stdout) ;
2471 }
2472 return rslt ;
2473 }
2474
2475 void ObjectMonitor::DeferredInitialize () {
2476 if (InitDone > 0) return ;
2477 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2478 while (InitDone != 1) ;
2479 return ;
2480 }
2481
2482 // One-shot global initialization ...
2483 // The initialization is idempotent, so we don't need locks.
2484 // In the future consider doing this via os::init_2().
2485 // SyncKnobs consist of <Key>=<Value> pairs in the style
2486 // of environment variables. Start by converting ':' to NUL.
2487
2488 if (SyncKnobs == NULL) SyncKnobs = "" ;
2489
2490 size_t sz = strlen (SyncKnobs) ;
2491 char * knobs = (char *) malloc (sz + 2) ;
2492 if (knobs == NULL) {
2493 vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ;
2494 guarantee (0, "invariant") ;
2495 }
2496 strcpy (knobs, SyncKnobs) ;
2497 knobs[sz+1] = 0 ;
2498 for (char * p = knobs ; *p ; p++) {
2499 if (*p == ':') *p = 0 ;
2500 }
2501
2502 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2503 SETKNOB(ReportSettings) ;
2504 SETKNOB(Verbose) ;
2505 SETKNOB(FixedSpin) ;
2506 SETKNOB(SpinLimit) ;
2507 SETKNOB(SpinBase) ;
2508 SETKNOB(SpinBackOff);
2509 SETKNOB(CASPenalty) ;
2510 SETKNOB(OXPenalty) ;
2511 SETKNOB(LogSpins) ;
2512 SETKNOB(SpinSetSucc) ;
2513 SETKNOB(SuccEnabled) ;
2514 SETKNOB(SuccRestrict) ;
2515 SETKNOB(Penalty) ;
2516 SETKNOB(Bonus) ;
2517 SETKNOB(BonusB) ;
2518 SETKNOB(Poverty) ;
2519 SETKNOB(SpinAfterFutile) ;
2520 SETKNOB(UsePause) ;
2521 SETKNOB(SpinEarly) ;
2522 SETKNOB(OState) ;
2523 SETKNOB(MaxSpinners) ;
2524 SETKNOB(PreSpin) ;
2525 SETKNOB(ExitPolicy) ;
2526 SETKNOB(QMode);
2527 SETKNOB(ResetEvent) ;
2528 SETKNOB(MoveNotifyee) ;
2529 SETKNOB(FastHSSEC) ;
2530 #undef SETKNOB
2531
2532 if (Knob_Verbose) {
2533 sanity_checks();
2534 }
2535
2536 if (os::is_MP()) {
2537 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2538 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2539 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2540 } else {
2541 Knob_SpinLimit = 0 ;
2542 Knob_SpinBase = 0 ;
2543 Knob_PreSpin = 0 ;
2544 Knob_FixedSpin = -1 ;
2545 }
2546
2547 if (Knob_LogSpins == 0) {
2548 ObjectMonitor::_sync_FailedSpins = NULL ;
2549 }
2550
2551 free (knobs) ;
2552 OrderAccess::fence() ;
2553 InitDone = 1 ;
2554 }
2555
2556 void ObjectMonitor::sanity_checks() {
2557 int error_cnt = 0;
2558 int warning_cnt = 0;
2559 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2560
2561 if (verbose) {
2562 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2563 sizeof(ObjectMonitor));
2564 }
2565
2566 uint cache_line_size = VM_Version::L1_data_cache_line_size();
2567 if (verbose) {
2568 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2569 }
2570
2571 ObjectMonitor dummy;
2572 u_char *addr_begin = (u_char*)&dummy;
2573 u_char *addr_header = (u_char*)&dummy._header;
2574 u_char *addr_owner = (u_char*)&dummy._owner;
2575
2576 uint offset_header = (uint)(addr_header - addr_begin);
2577 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2578
2579 uint offset_owner = (uint)(addr_owner - addr_begin);
2580 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2581
2582 if ((uint)(addr_header - addr_begin) != 0) {
2583 tty->print_cr("ERROR: offset(_header) must be zero (0).");
2584 error_cnt++;
2585 }
2586
2587 if (cache_line_size != 0) {
2588 // We were able to determine the L1 data cache line size so
2589 // do some cache line specific sanity checks
2590
2591 if ((offset_owner - offset_header) < cache_line_size) {
2592 tty->print_cr("WARNING: the _header and _owner fields are closer "
2593 "than a cache line which permits false sharing.");
2594 warning_cnt++;
2595 }
2596
2597 if ((sizeof(ObjectMonitor) % cache_line_size) != 0) {
2598 tty->print_cr("WARNING: ObjectMonitor size is not a multiple of "
2599 "a cache line which permits false sharing.");
2600 warning_cnt++;
2601 }
2602 }
2603
2604 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2605 &warning_cnt);
2606
2607 if (verbose || error_cnt != 0 || warning_cnt != 0) {
2608 tty->print_cr("INFO: error_cnt=%d", error_cnt);
2609 tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2610 }
2611
2612 guarantee(error_cnt == 0,
2613 "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2614 }
2615
2616 #ifndef PRODUCT
2617 void ObjectMonitor::verify() {
2618 }
2619
2620 void ObjectMonitor::print() {
2621 }
2622 #endif